1. Field of the Invention
The invention relates to an electronic circuit and method for enhancing an image, in particular, for television applications.
2. Description of the Related Art
FIG. 4 shows such a circuit known in the art, e.g., substantially from U.S. Pat. No. 5,903,680. The circuit comprises a high-pass filter 110′ for separating a high-frequency component from an image signal, representing an image pixel Pn of the image. The image signal as well as at least one further image signal representing a neighbor pixel Pn−j, Pn+k, being located in a N-dimensional environment of said image pixel, are low-pass filtered by low-pass filters 120-2, 120-1, 120-3, respectively. The low-pass-filtered image signal is further filtered by a N-dimensional image enhancement filter 130 under consideration of the low-pass-filtered further image signals in order to generate an intermediate image signal. Finally, the intermediate image signal and the high-frequency component are mathematically combined by a combining element 150 in order to generate an enhanced image output signal of the electronic circuit.
The circuit shown in FIG. 4 implements a concept of how to improve the subjective quality of the output signal of the image enhancement filter 130. More specifically, according to FIG. 4, only low-frequency components of the image signal and of the further image signals are input to the N-dimensional image enhancement filter 130, while the high frequency component is bypassed. This is done because filtering of only the low frequency component is more effective than carrying out filtering on the entire spectrum of the image signals. Moreover, adding the bypassed high frequency component to the intermediate output signal of the image enhancement filter 130 improves the dynamic behavior of the whole electronic circuit and reduces the impression of blurring, which is prevalent in spatial image enhancement filters having low-pass characteristic. Further, by applying this concept, the impression of sharpness in the image input to the image enhancement filter 130 is preserved in the image after being processed by the image enhancement filter.
However, the circuit according to FIG. 4 and the described concept, have the disadvantage that they do not work economically if the input image signals are scanned in a noncontiguous fashion or in any direction other than parallel or orthogonal to the horizontal axis. In particular, when a 2- or 3 or, in general, N-dimensional image enhancement filter is used, the costs become prohibitive, since then the low-pass filtering must be done for each tap of the filter, that is for the image signal as well as for each of the further image signals.